The invention relates to a method for suppressing electromagnetic background radiation in an image according to a precharacterization, a device for carrying out the method according to a precharacterization, a measuring instrument, and a use of the device for identifying a laser signal as a mark for subsequent measuring processes as well as for identifying a source of a laser signal.
The detection of individual signals or complete signatures of electromagnetic radiation has a wide range of uses. It is possible to make a distinction between two fundamental objects.
On the one hand, a radiation source (target source) or the real or virtual image thereof is to be detected and its position or its direction determined. Applications exist here, for example, in the automatic orientation of measuring instruments or the detection of radiation sources in the military sector (for example, for detecting an opponent's target illumination or distance measurement, as part of an active protection system). Other applications are the tracking of laser radiation for controlling autonomous vehicles or the detection and tracking of astronomical objects. The emitted radiation can in principle have a continuous spectrum or a line spectrum. However, the detection of a source requires information about its spectral characteristic, which may be relatively constant, for example, for lasers but are subject to greater uncertainties for thermal emitters, owing to the temperature-dependent emission.
On the other hand, the position of objects should be detectable by marks (target illumination), for example, by means of a light spot or a reflector which, in the case of divergent irradiation, is detectable on the basis of its high reflectivity. Thus, for example, in the geodetic area for surveying purposes, the position of reflecting points which have, for example, prism reflectors, corner cubes or reflector foils can be precisely determined. For this purpose, a laser signal is emitted (target illumination) and the direction from which any reflected radiation arrives is determined. This makes it possible to mark, or to make, markable, specific points, for example, on machines or buildings, by mounting a reflector and to measure their position automatically.
In geodetic applications, for example, a search by means of a laser spot is used, together with a theodolite, for determining the direction in which a reflector (prism arrangement) is present. For this purpose, a laser signal is emitted by a theodolite. If this signal is incident on a reflector prism, the laser beam is reflected back and can be received again in the theodolite. A similar application is the guidance of automatic robot systems by light marks or reflecting marks in the production sector.
Problems with the detection and position determination of electromagnetic signals are presented in particular by existent interfering radiation, such as, for example, the background radiation of the daytime sky, direct sunlight, interior lighting of rooms or thermal emitters, such as, for example, metallic melts in industrial applications.
In the case of unfavourable distance conditions, such interfering radiation may be stronger than the signal to be measured. Particularly when lasers are used as signal sources, problems arise since, for safety reasons, especially for the protection of the human eye, their power is not permitted to exceed certain values.
The influence of the interfering radiation is eliminated in methods of the prior art by a light/dark differential image method. For this purpose, one measurement is carried out with the signal source (target source or target illuminator) switched on and a further measurement is carried out with the signal source switched off. The signal, for example, a laser spot or the radiation returning from a reflecting mark, can be extracted from the difference between the two measurements.
A corresponding device in the form of a camera system with light having controlled distance measurement for tele-robotic applications is disclosed in the Patents U.S. Pat. No. 5,673,082 and 5,684,531. By means of the camera system, images are recorded with a laser switched on and switched off. A comparison of the images which is carried out pixel by pixel and in which identical pixels are eliminated from the image leads to a differential image which should include only the laser spot.
The U.S. Pat. No. 5,901,236 describes a method and a device for position measurement by image processing. Here, an object to be measured is provided with a light-emitting source, for example, a light-emitting diode (LED), which is periodically switched on and off. Four successive images of the object are recorded with an exposure time of a quarter of the period of the light-emitting source and differential images generated in each case from the first and third and from the second and fourth image. The differential image having the greatest brightness is used for the position measurement. This procedure is intended to ensure that the switched-on and switched-off states are subtracted from one another for at least one of the two differential images.
For exact position determination at distances of more than 50 m, known methods require the use of an arrangement (array) of a plurality of sensors for the detection. Examples of such sensor arrays are large-area sensors, such as CCD or CMOS camera sensors and CMOS- or CCD-based line sensors. An image of a three-dimensional region (field of view) to be evaluated is generated by an optical system on the sensor array and is converted into a signal by the sensor array.
If a signal to be detected is present in the field of view, said signal is registered, for example, as a bright point of a laser spot, in the image. In contract to individual sensors, for example, position sensitive device (PSD) (large-area diodes which determine the position of the centre of gravity of a light beam incident on their sensor surface), in the case of sensor arrays the interfering radiation incident, optionally from the entire environment, is distributed over a multiplicity of part-sensors (pixels) of the array. This division results in a smaller interfering signal and hence an improved signal/noise ratio. This increases the reliability of the signal detection and enhances the resolution of the position determination. Moreover, in the case of individual sensors, the dynamics of the sensor is exceeded by intense interfering radiation and hence a measurement is impossible. For example, the recording of the sun in the image section rapidly leads to supersaturation of the sensor.
However, the increased positional resolution of sensor arrays is offset by the disadvantage of a substantially increased time requirement for a measurement in comparison with individual sensors. This increased time requirement is due to the numerous part-sensors of the sensor array which are to be evaluated for a measurement.
If it is intended to achieve a positional accuracy in the sub-10 angular second range, this results in a lower limit for the number of individual sensors in the sensor array, with a given constant size of the field of view. In some cases, less than 200 images per second can be read out from sensor arrays of the lower to middle segment which are suitable for this purpose.
When the light/dark differential image method is used, this comparatively long measuring time gives rise to the problem of the change of an environment to be considered during the measuring time.
For example, atmospheric turbulences during the measuring time decisively influence the apparent position of remote objects. Moreover, it may be desirable to perform scan movements during the measuring process with the measuring arrangement in order to be able to evaluate a greater direction range. This too results in a rapid change in the image content. In addition, moving interfering objects, for example, traveling motor vehicles, can greatly change the image content during the measuring process.
If, apart from the signal to be analysed, the image content changes decisively between light image and dark image, the signal can no longer be extracted without errors from the difference between the two images. This faulty signal in turn gives rise to errors in the detection and in the determination of the signal position.
It is therefore the object of the invention to provide a method and a device by means of which a detection and position determination of an electromagnetic signal in a considered field of view is improved.
It is a further object to ensure safe and reliable identification of the signal also in the case of a strong or moving interfering radiation background or in the case of changes in the field of view.
These objects are achieved, according to the invention, by the characterizing features described in this specification. Advantageous and alternative embodiments and further developments of the method and of the device are evident from the features of the subclaims.